Well logging is a common practice in the oil and gas industry to evaluate underground formations for the presence and producibility of hydrocarbon reservoirs. Among the most important parameters determined in the process are the depth and thickness of formation layers containing a potential hydrocarbon reservoir, the formation porosity (i.e., the relative amount of void space in the formation), the hydrocarbon saturation (i.e., the relative percentage of hydrocarbons versus water in the pore space), and the permeability (i.e., the ability of the oil, gas, or water to flow out of the formation, into the well and eventually to the surface for recovery).
Presently, nuclear magnetic resonance (NMR) well logging is considered to be one of the most effective technique for determining these geologic parameters. NMR technology has many advantages over other logging techniques (such as gamma ray logging, sonic logging, electric logging, and others), one of the most significant being the independence of NMR measurements from formation lithology. In particular, NMR data relates in a simple manner to formation pore sizes. This relationship facilitates detection of formation fluids (i.e., gas, oil, and water) independent of the matrix mineralogy. To this end, in addition to estimation of formation porosity, hydrocarbon saturation and permeability, NMR logging enables computation of clay-bound water, capillary-bound water, and free fluid volumes, which are essential to comprehensive formation evaluation.
Generally, NMR measurements are performed as follows. A downhole static magnetic field B0 is used to align the magnetic moment of spinning hydrogen (H) protons in the formation in the direction of the B0 magnetic field. In order to establish thermal equilibrium, the hydrogen protons must be exposed to the polarizing field for a multiple of the characteristic relaxation time T1. Then, the magnetic component of a radio frequency (RF) electromagnetic pulse polarized in a second direction orthogonal to the static field B0 is used to tip the protons to align them in a third direction that is orthogonal to both the first and the second direction. This initial RF pulse is known as a 90° pulse. Following the 90° pulse the protons in the formation begin to precess about the axis of the first direction. As a result, the protons produce an oscillating magnetic field, having a frequency directly proportional to the B0 field intensity at the proton's location. Due to inhomogeneities in the static magnetic field and irreversible molecular processes, the protons quickly begin to de-phase, which causes the induced signal to decay. Nevertheless, the dephasing process is partially reversible. In particular, by applying an 180° RF pulse, the instantaneous phases are reversed such that the protons gradually come back into phase, thus rebuilding the induced signal. After the signal peaks at the time when the protons are back in phase, the signal will begin to decay again due to dephasing in the opposite direction. Another 180° RF pulse can be used to again reverse the instantaneous phases and thereby rebuild the signal.
By using a series of 180° RF pulses, the signal is periodically rebuilt after each dephasing, although each rebuilding is to a slightly lesser peak amplitude due to the irreversible molecular processes so eventually it dies out completely. Each rebuilding of the signal in this manner is called a spin echo, and the time constant associated with the decay of the spin echo amplitudes is known as the transverse relaxation time T2. A particular sequence of pulses, known in the art as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, is most frequently used. For a more comprehensive overview of the NMR technology including logging methods and various tool designs, the interested reader is directed, for example, to the book by Coates et al. entitled “NMR Logging: Principles and Applications” distributed by Gulf Publishing Company (2000), and incorporated in its entirety herein by reference for background. Additional description of NMR logging techniques is provided, for example, in U.S. Pat. Nos. 4,710,713; 4,717,876; 4,717,877; 4,717,878; 4,939,648; 5,055,787; 5,055,788; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164; 6,051,973; 6,107,796; 6,111,408; 6,242,913; 6,255,819; 6,268,726; 6,362,619; 6,512,371; 6,525,534; 6,531,868; 6,541,969; 6,577,125; 6,583,621, 6,646,437 and 6,717,404, which are incorporated herein by reference.
NMR logging is typically performed using wireline tool or logging-while-drilling (LWD) tools. In the conventional wireline-logging technology, NMR logging is performed as the logging tool is being lowered into a drilled borehole. In the emerging LWD technology, the logging tools are generally rigged up as a part of the drilling string and follow a drill bit during actual well drilling. Each tool type has its own advantages. The wireline-tools enable high logging speeds and high-quality measurements. The LWD tools, on the other hand, provide real-time data during drilling operations that may be used to prevent loss of circulation, blowouts, stuck pipes, hole instability and other disastrous consequences of borehole drilling.
Yet another significant benefit of LWD technology is that it facilitates directional drilling of the borehole. Specifically, directional drilling involves the drilling of a well bore along a deviated course in order to reach a target region at a particular vertical and horizontal distance from the original surface location. This form of drilling is particularly useful for pay zone steering: a procedure in which directional drilling is used to obtain an appropriate wellbore trajectory into an oil producing formation bed (or “pay zone”) based on real-time formation evaluation data and then drill substantially within pay zone boundaries. Directional drilling may be used to penetrate multiple pay zones by using fewer wells, as well as increase the borehole volume and flow rates in the pay zone.
Notwithstanding the numerous advantages of current NMR technology, present generation of NMR tools have one key weakness—shallow depth of investigation—which is typically about 10-20 cm from the tool. This is a problem because producible formation fluids (e.g., gas, oil, and water) are often displaced in the formation surrounding the borehole by invading borehole fluids (i.e., drilling mud) driven by high borehole pressure. Such invasion may occur as far as one meter into the formation with wide variations due to fluid composition, formation permeability, and applied pressure difference. As a result, conventional NMR tools having shallow depths of investigation receive signals only from the invaded section of the formation. Measurements at such shallow depths are useful to replicate porosity, T1 and T2 relaxation measurements, type and volume of bound fluid, and volume available for producible fluids. Because of displacement of the formation fluids, the NMR LWD systems, however, cannot accurately quantify the amounts of producible hydrocarbons (i.e., oil and gas) present in the formation surrounding the borehole—a factor of great significance in predicting producibility of a hydrocarbon reservoir.
The main obstacle to conducting deep NMR measurements is the high gradient G0 of the static magnetic field B0. In other words, the strength of the magnetic field B0 falls off very rapidly with increasing distance from the tool. Such decrease in the magnetic field strength is primarily attributed to the magnetic configuration of the NMR tool. For instance, U.S. Pat. No. 4,350,955 to Jackson et al. (“Jackson et al.”) discloses a NMR apparatus comprising a pair of cylindrical permanent magnets placed co-axially with like poles facing each other and a loop antenna placed between the magnets for transmitting and receiving radio signals. The opposing magnetic fields combine to form a toroidal region of relatively homogeneous radial static magnetic field B0. The distance of the homogeneous field region from the axis of the magnets depends on the magnet dimensions and their separation. The closer are the magnets the stronger is the combined magnetic field. The magnetic field lines in the Jackson et al. design, however, disperse very rapidly in the relative proximity of the tool and therefore provide a low magnetic field gradient G0 only at a distance of about 10 cm from the tool.
An improvement of the Jackson's et al. tool design is disclosed in U.S. Pat. No. 4,629,986 to Clow et al. (hereinafter “Clow et al.”). Clow et al. placed a highly permeable ferromagnetic material between two permanent magnets, positioned as in Jackson's layout. This ferromagnetic material shunts more magnetic flux into the center of the tool and produces radial magnetic field lines radiating in vertical planes. Contrary to Jackson's et al. design, this configuration keeps the magnetic field lines more focused and parallel at a greater distance, which results in a stronger field B0 and low gradient G0 further away from the tool (i.e., about 20 cm). However, such investigation depths are still too shallow to enable adequate NMR measurements in regions of the formation unaffected by the invaded formation fluids.
Another prior art design is described, for example, in U.S. Pat. No. 6,246,236 to Poitzsch et al. (hereinafter “Poitzsch et al.”), claiming priority to U.S. Pat. No. 5,977,768 to Sezginer et al., which discloses NMR tool having a low-gradient sonde and a high-gradient sonde positioned in tandem along the longitudinal axis of a tool. The '236 and '768 patents are incorporated herein by reference. The low-gradient sonde comprises two permanent magnets having separation of about 65 cm and an interposed magnetically permeable member. The configuration provides a relatively weak magnetic field B0′, which has low (approximately 3 G/cm) gradient G0′ that is measured at a distance of approximately 20 cm radially from the tool. The second sonde comprises two permanent magnets about 20 cm apart with an interposed magnetically permeable member. This configuration provides a stronger magnetic field B0″ at approximately the same distance from the tool as the low-gradient sonde, but with greater gradient G0″ (approximately 10-20 G/cm).
Each magnetic configuration in Poitzsch et al. has its own advantages. NMR measurements performed in the low gradient region, for instance, are less sensitive to the lateral motion of the tool than the measurements in the high gradient region—a characteristic useful in LWD applications, in which drill string typically undergoes severe vibrations. High field strength in high gradient region, on the other hand, provides better signal-to-noise ratio (SNR), which is very important in both wireline and LWD applications. The Poitzsch et al. tool, however, conducts NMR measurements in shallow volumes (about 20 cm deep), which are typically invaded by borehole fluids. As a result, information gathered by the tool is limited and the quality of its measurements may be compromised. Moreover, none of the above tools provide directionally sensitive data about the formation, which would facilitate directional drilling capabilities.
Sezginer et al. use a single transmitter that powers all antennas in transmit mode. That means that (a) all antennas have to be tuned to the same frequency, and (b) the interaction between antennas (mutual detuning) has to be negligible. It is not possible to suppress the mutual interaction while at the same time maintaining good azimuthal coverage. Thus, Sezginer et al. require either very narrow antennas, which have low SNR and poor azimuthal coverage or do not disclose a workable system due to the fact that antennas in close proximity are electrically equivalent to coupled tank circuits, which exhibit split resonances. The prior art does not disclose or suggest a system that obtains directional information from a formation and also is capable of operating at multiple frequencies.
Accordingly, it is an object of the present invention to provide a NMR tool suitable for comprehensive evaluation of underground formations during wireline or LWD operations. In particular, it is an object of the invention to enable NMR measurements in deep regions of the formation that are substantially free of borehole fluids invasion. Another object is to enable both shallow and deep measurements using a single magnetic assembly. Yet another object of the invention is to provide NMR tool having directional sensitivity and suitable for directional drilling based on directionally sensitive NMR measurements. A further object of the invention is to provide an NMR pulse sequence that minimizes the tool's power consumption, while maximizing the SNR of deep NMR measurements.